The present invention relates to a method for ensuring the braking effect of brakes of vehicles under wet conditions.
Travelling for relatively long periods without braking and wet conditions as a result of rain and/or plumes of spray from vehicles in front may lead to a delayed response of brakes or to steering reactions or offsets of vehicles, depending on whether the braking effects on the two sides of a vehicle axle are symmetrical or asymmetric. This is based on the fact that, when the brakes are actuated, water first evaporates between the brake lining and brake disc. The brake achieves its unrestricted intended effect only when this water has evaporated completely.
In order to guarantee a good response of brakes even under wet conditions, flow-off channels for the water have been provided either on the brake lining or on the brake disc or brake drum. For this purpose, for example, grooves are made on the brake linings. After even only a short time, however, these grooves become clogged with abrasive particles, so that their effect is no longer guaranteed. Particularly in the case of motor cycles, it is therefore customary to use perforated brake discs. However, the service life of such perforated brake discs is shorter than that of comparable non-perforated brake discs. Moreover, perforated brake discs also lead to an increased wear of the brake linings.
Furthermore, DE 4,101,759 5 A1 describes the use of the actuation of the screen wipers as a sensor device for wet conditions. By way of a signal from this sensor device, the brake-actuation of a headway control system is adapted to the wet road state in which there is reduced adhesion between the wheel and roadway. EP 0,486,340 B1 describes an arrangement which makes it possible to generate brake pressure via a brake booster but without causing a movement of the brake pedal. A brake system having the necessary devices for generating a brake pressure, without an actuation of the brake pedal, is also described in D 4,329,139 C1.
An object of the present invention is to ensure that, even under wet conditions, the unrestricted intended braking effect is always achieved on wheels of a vehicle brake system. At the same time, the service life of the brake system is not to be appreciably influenced.
The foregoing object has been achieved in that, when a sensor device detects that wet conditions influencing the braking effect are present and, if it has been found that the braking effect of a brake could be impaired by correspondingly long travelling without braking, a temporary braking operation is carried out such that the deceleration of the vehicle is not perceived by the driver.
As a result of the actuation of the brake, the water precipitated on the brake evaporates. The immediate full operating capacity of the brake is thereby ensured until a specific quantity of water has been precipitated on the brake disc again. When this is so, a braking operation once again takes place.
According to one embodiment of the invention, the screen-wiper switch is used as a sensor device. The screen-wiper switch is a particularly simple and cost-effective sensor device because it is present in any event in the vehicle. In the simplest version, a distinction is made only between the states "screen wiper inoperative" and "screen wiper operative". In another version, there can then also be a distinction between the individual operating modes of the screen wiper (for example, interval control, slow and fast wiping speed) and, at the same time, the frequency and/or intensity or duration of the braking operation can be influenced.
According to further embodiments of the invention, the vehicle deceleration occurring as a result of this braking operation can be determined or estimated. The drive torque generated by the engine can be regulated or determined via a regulating device so that, during this braking operation, no actual vehicle deceleration resulting from the automatic braking operation occurs. The vehicle deceleration taking place on account of the braking operation can thus be compensated. Action on the vehicle brake system remains concealed from the driver. Even without an active compensation of the braking effect via the drive, a deceleration sufficient for the intended function of the method can scarcely be perceived by most drivers.
In order to ensure the directional stability of the vehicle, it is advantageous to carry out the braking operation on the front axle only. Where passenger cars are concerned, in particular, it is necessary to carry out the braking operation on the front axle on account of the higher load caused by rain, splashes of water and water spray. The braking operation on the rear axle can be dispensed with.
By use of the term "brakes" it is understood to include all types of vehicle brakes in which the braking effect is generated by friction between a brake lining and a braking surface. Since brake discs are particularly exposed to the wet and are not as well protected or closed off as drum brakes, use of the method of the present invention in disc brakes particularly is advantageous.
A further appropriate alternative is to record the temperature of the brake disc or brake drum and to carry out the braking operation when the brake-disc temperature falls below a temperature limit value. The brake-disc temperature does not necessarily have to be measured, but, for example, it can also be determined computationally, inasmuch as the heat generated as a result of all the braking operations and the heat transmitted into the environment by the discs are continuously computed and summed up.